Composition and process for treating magnesium-containing metals and product therefrom

ABSTRACT

The present invention produces a highly corrosion-resistant, highly rust-inhibiting, and strongly paint-adherent conversion coating on the surface of magnesium-containing metals, and does so rapidly and at relatively low temperatures, using a substantially chromium-free conversion treatment bath.

TECHNICAL FIELD

The invention relates to a conversion treatment bath composition(hereinafter usually called either "bath" or "composition", either ofwhich in this context is to be understood as meaning "bath composition")that is suitable, either as such or after dilution with water, forimproving the corrosion resistance and rust-inhibiting performance of,and the adherence of paint to, the surface of magnesium-containingmetals by forming a conversion coating thereon by contact at suitabletemperatures for suitable times. The invention also relates to aconversion treatment method that uses this bath and to objects to whichthis conversion treatment has been applied.

BACKGROUND ART

The conversion treatment of magnesium-containing metals is alreadyknown, for example, from JIS H-8651, MIL-M-3171, and so forth, and theseconversion treatments have found practical application as underpaintcoating treatments for magnesium-containing metals. However, all ofthese conversion treatment baths contain hexavalent chromium ions. Sincehexavalent chromium ions are a pollution source, their presence isaccompanied by a number of problems, for example, processing of theeffluent from conversion treatment, management of the workingenvironment, and the like.

These conversion treatments are also subject to other problems; forexample, they require high treatment temperatures and long treatmenttimes, and their treatment baths are very concentrated. Moreover, theconversion films produced by these conversion treatments have a strongtendency to be nonuniform.

Japanese Patent Publication Number Hei 3-6994 [6,994/1991] discloses aconversion treatment for magnesium-containing metals that does not usehexavalent chromium ions. This conversion treatment takes the form of aCr⁶⁺ -free phosphate conversion treatment, which, however, is notsufficient to provide magnesium-containing metals with corrosionresistance. Specifically, after the phosphate treatment step, theoverall treatment method of Japanese Patent Publication Number Hei3-6994 teaches treatment with silicate and then silicone. The phosphateconversion coating by itself provides only a poor corrosion resistanceand adherence when applied to the surface of magnesium-containing metalsas an underpaint coating treatment. This treatment method also suffersfrom other problems; for example, it requires a multistep treatmentprocess, high treatment temperatures, and long treatment times.

The use of treatment baths based on zinc phosphate, iron phosphate,zirconium phosphate, and so forth, is already known within the realm ofphosphate-based conversion treatment methods, but these methodsessentially can not provide the surface of magnesium-containing metalswith a corrosion resistance satisfactory for practical applications.

A manganese phosphate treatment is reported in Section 7 of JapaneseIndustrial Standards ("JIS") H-8651, but this treatment bath isunsuitable for practical applications because it uses chromium, requireshigh treatment temperatures of 80° C. to 90° C., and requires longtreatment times of 30 to 60 minutes.

DISCLOSURE OF THE INVENTION

Problems to Be Solved by the Invention

Thus, as described above, the following problems are associated withconversion treatment methods according to the prior art formagnesium-containing metals:

1) the use of chromium, which is a typical pollution source;

2) a requirement for high treatment temperatures;

3) a requirement for long treatment times;

4) in the case of chromium-free systems, the resulting conversioncoating has a corrosion resistance and paint adherence inferior to thoseobtained with the use of chromium;

5) management of the treatment bath is difficult, making it difficult toconsistently obtain a uniform conversion coating; and

6) degradation of the working environment.

The invention was pursued in order to solve the problems listed above.The present invention takes as an object the introduction of asubstantially chromium-free conversion treatment bath composition thatis able to form a uniform, highly corrosion-resistant, highlyrust-inhibiting, and strongly paint-adherent conversion film on thesurface of magnesium-containing metals. An additional object of theinvention is that said conversion film should be formed by a rapid,low-temperature, and low-cost method in which bath management is simpleand which uses relatively simple equipment. Other objects of theinvention are a conversion treatment method that uses said compositionand magnesium-containing materials that have been conversion treatedwith said composition.

SUMMARY OF THE INVENTION

The aqueous liquid conversion treatment bath composition of theinvention, which is intended for application to magnesium-containingmetals, characteristically is an aqueous solution that has a pH of 2.0to 5.0 and contains phosphorus-containing acid, divalent manganese ions(hereinafter usually described simply as "manganese ions"), and at leastone amine. Alternative embodiments of the invention are compositionsready for use, called "working compositions", and concentrates, fromwhich working compositions can be made by dilution with water only.

The amine compound used in the conversion treatment bath composition ofthe invention is preferably selected from aliphatic amine compounds,heterocyclic amine compounds, and aromatic amine compounds.

The conversion treatment bath composition of the invention may alsocontain one or more selections from the group comprising nitrate ions,sulfate ions, and fluorine-containing compounds.

The method of the invention for the conversion treatment ofmagnesium-containing metals characteristically consists of forming aconversion coating that contains phosphorus-manganese andmanganese-nitrogen and/or other nitrogen compounds on the surface ofmagnesium-containing metal by contacting said magnesium-containing metalwith an aqueous conversion treatment bath that has a pH of 2.0 to 5.0and contains phosphoric acid, manganese ions, and amine(s).

Conversion-treated magnesium-containing metal in accordance with thepresent invention characteristically comprises a magnesium-containingmetal substrate whose surface is at least partially covered with aconversion coating that contains phosphorus-manganese andmanganese-nitrogen and/or other nitrogen compounds and that has beenformed by contacting the surface of said substrate with an aqueousconversion treatment bath that has a pH of 2.0 to 5.0 and containsphosphorus-containing acid, manganese ions, and amine(s).

DESCRIPTION OF PREFERRED EMBODIMENTS

The conversion coatings on conversion-treated material in accordancewith the invention preferably contain 1 to 500 milligrams per squaremeter (hereinafter usually abbreviated as "mg/m² ") of manganese and 1to 1000 mg/m² of phosphorus. In addition and independently, theseconversion coatings preferably contain a large number of reticulatingcracks having widths of 0.1 to 2 micrometers.

Magnesium-containing metals encompassed by the invention include puremagnesium and alloys containing at least 50% magnesium, for example,Mg-Al-Zn alloys, Mg-Zn alloys, Mg-Al-Zn-Mn alloys, and the like. Themagnesium containing metals preferably contain, with increasingpreference in the order given, at least 55, 65, 75, 80, 85, 90, or 95%by weight of magnesium.

The phosphorus-containing acid used in the invention preferablycomprises at least one selection from metaphosphoric acid,orthophosphoric acid, condensed phosphoric acids, phosphorous acid,hypophosphorous acid, and the like; the use of orthophosphoric acid ismost preferred. The phosphorus-containing acid also functions as etchantfor the magnesium-containing metal and is thus effective for the actualproduction of the conversion coating. The concentration in working bathsof phosphorus from these free acids and/or anions derivable byionization of these acids, including any phosphorus containing anionsadded to the baths in the form of salts, preferably is, with increasingpreference in the order given, at least 0.01, 0.02, 0.04, 0.08, 0.16,0.20, 0.24, 0.28, 0.32, 0.34, 0.35, 0.36, or 0.37 gram-atoms per liter(hereinafter usually abbreviated "g-a/L") and independently preferablyis, with increasing preference in the order given, not more than 1.2,1.0, 0.90, 0.80, 0.70, 0.65, 0.60, 0.58, 0.56, 0.55, 0.54, or 0.53g-a/L.

Manganese ions can be supplied by, for example, manganese dihydrogenphosphate, Mn(H₂ PO₄)₂ ·4H₂ O; manganese hydrogen phosphate, MnHPO₄ ·H₂O; manganese nitrate, Mn(NO₃)₂ ·xH₂ O; manganese sulfate, MnSO₄ ·H₂ O;manganese fluoborate, Mn(BF₄)₂ ·6H₂ O; manganese carbonate, MnCO₃ ; andthe like. Generally, in order to keep the composition of the treatmentbath according to the invention as simple as possible, the use of one ormore of the above noted manganese orthophosphate salts is preferred,because this leads to preferred ratios between manganese and phosphoruscontents and provides a buffering action that helps maintain the pH ofthe composition within the desired range. The manganese ions arebelieved to be the source of the manganese compound present in theconversion coating formed on the surface of the magnesium-containingmetal treated according to the invention. The manganese ions therein arebelieved to act to provide the conversion coating with an excellentcorrosion resistance and rust inhibition and to improve the paintadherence. The concentration of manganese ions (assuming totalionization of any manganese salts present) in working baths according tothe invention preferably is, with increasing preference in the ordergiven, at least 0.005, 0.008, 0.016, 0.030, 0.040, 0.050, 0.055, 0.060,0.065, 0.068, 0.072, 0.074, 0.075, 0.076, or 0.077 g-a/L andindependently preferably is, with increasing preference in the ordergiven, not more than 1.0, 0.5, 0.4, 0.30, 0.25, 0.20, 0.18, 0.16, 0.14,0.13, 0.12, or 0.11 g-a/L. Independently, the ratio of the concentrationin g-a/L of manganese to that of phosphorus preferably is, withincreasing preference in the order given, at least 0.02, 0.04, 0.08,0.10, 0.12, 0.14, 0.16, 0.18, 0.19, or 0.20 and independently preferablyis, with increasing preference in the order given, not more than 1.0,0.7, 0.50, 0.40, 0.35, 0.30, 0.28, 0.27, 0.26, 0.25, 0.24, 0.23, or0.22. These ratios, unlike the absolute concentration values givenabove, apply to concentrates as well as to working baths.

The concentration in working treatment baths according to the inventionof the amine component preferably is, with increasing preference in theorder given, at least 0.01, 0.020, 0.030, 0.050, 0.070, 0.090, 0.110,0.130, 0.150, 0.170, 0.180, 0.185, or 0.190 gram moles per liter(hereinafter usually abbreviated "molar" or "M") and independentlypreferably is, with increasing preference in the order given, not morethan 1.0, 0.90, 0.80, 0.70, 0.60, 0.50, 0.40, 0.320, 0.280, 0.260,0.240, 0.230, 0.220, 0.210, or 0.200M. Also independently, for bothconcentrates and working baths, the ratio of the total molarconcentration of amine to the concentration of manganese in g-a/L asdefined above preferably is, with increasing preference in the ordergiven, at least 0.2, 0.4, 0.6, 0.8, 1.0, 1.20, 1.30, 1.40, 1.50, 1.60,1.70, 1.80, 1.90, 2.00, 2.10, 2.20, 2.30, or 2.40 and independentlypreferably is, with increasing preference in the order given, not morethan 5.0, 4.5, 4.0, 3.5, 3.2, 3.0, 2.9, 2.8, 2.7, 2.6, or 2.5.

The amine component used by the present invention is preferably selectedfrom those aliphatic amine compounds, heterocyclic amine compounds, andaromatic amine compounds that are soluble in an aqueous solution at pH2.0 to 5.0 and at a temperature of 25° C. to an extent of at least, withincreasing preference in the order given, 10, 9, 8, 7, 6, 5, 4, 3, 2,1.0, 0.8, 0.6, 0.5, or 0.4% by weight. Aliphatic amine compounds of thistype are exemplified by propylamine, diethylamine, and triethylamine.The heterocyclic amines and aromatic amines that fall into this categoryare exemplified by triazole and aniline, respectively. Triethylamine isparticularly preferred.

The presence of an amine component in the treatment bath composition isa crucial feature of the present invention. The presence of aminecompound in a conversion treatment bath that contains phosphoric acidand manganese ion is believed to serve to prevent excessive etching ofthe surface of magnesium-containing metal. The resulting optimal etchmakes possible the reliable production of a product that has theexcellent corrosion resistance, rust inhibition, and adherence desiredof an undercoating composition for application with paints and syntheticorganic resins.

The inventors have also discovered that major improvements in corrosionresistance, rust inhibition, and adherence for paint and the like, areobtained when manganese-nitrogen compounds, e.g., manganese nitride andthe like, are present along with manganese phosphate in the conversioncoating formed on the surface of magnesium-containing metal.

The presence of these compounds in the conversion coating of theinvention can be determined by X-ray diffraction (Hanawalt method:comparison of the X-ray diffraction angles and intensities with JointCommittee on Powder Diffraction Standards ("JCPDS") cards).

The conversion treatment bath composition according to the presentinvention should have a pH of 2.0 to 5.0. Etching by thephosphorus-containing acid is too severe when the pH is less than 2.0.This causes the adherence of smut on the resulting conversion coating,which reduces the improvement in its corrosion resistance, and causeslarge fluctuations in the bath. Etching by the phosphorus-containingacid is too weak at a pH above 5.0. This causes a thin conversioncoating formation and prevents the appearance of the reticulatingcracks, and thereby causes problems such as a reduction in thepost-painting secondary adhesion and the like.

The conversion treatment bath composition of the invention may alsocontain one or more selections from the group comprising nitrate ions,sulfate ions, and fluorine-containing compounds. This component is usedto optimize etching.

The conversion treatment bath composition of the invention may alsocontain ions or compounds of Mg, Al, Zn, Ca, Ba, Sn, Zr, and Si. On theother hand, the content of Cu, Ni, and Fe is preferably kept as small aspossible because these elements exercise a corrosion-acceleratingactivity on magnesium-containing metals. More particularly,independently for each component noted, the concentration in treatmentbaths according to the invention of each of copper, nickel, and ironpreferably is, with increasing preference in the order given, not morethan 0.1, 0.01, 0.005, 0.001, 0.0005, 0.0001, 0.00005, 0.00001,0.000005, 0.000001, 0.0000005, or 0.0000001 g-a/L.

A method of the invention includes forming a conversion coating thatcontains phosphorus-manganese and manganese-nitrogen compounds on thesurface of magnesium-containing metal by contacting the surface of saidmagnesium-containing metal with an aqueous conversion treatment baththat has a pH of 2.0 to 5.0 and contains phosphorus-containing acid,manganese ions, and amine(s). Network-forming or reticulating grooves(cracks) having widths of 0.1 to 2 micrometers are preferably producedin this conversion coating layer. These reticulating grooves in theconversion coating are believed to have an excellent anchoring effectfor paint films and yield a major improvement in paint film adherence.

Conversion treatment according to the method of the present invention isgenerally done at a relatively low temperature of 20° C. to 65° C., andwith a relatively short treatment time of 0.2 to 6 minutes; the time ofcontact preferably is, with increasing preference in the order given, atleast 0.5, 1, or 2 min.

The method of the invention as described above can form conversioncoatings with thicknesses of 0.1 to 3.0 micrometers, and theseconversion coatings appear amorphous in character to visual examination,even at a magnification of 1000×. However, as noted below, the coatingsproduce X-ray diffraction patterns indicative of somemicrocrystallinity.

Pretreatment of the magnesium-containing metal prior to application ofthe conversion treatment of the invention may include an alkali etch inaddition to the usual cleaning procedures. This alkali etchpreferentially removes alloy components, such as Al, Zn, and so forth,that segregate onto the surface of magnesium-containing metals, and thussupports a smooth and efficient etch of the magnesium during conversiontreatment and thereby accelerates formation of the conversion coating.In addition, the alkali etch functions to increase the paint adherenceof the conversion coating by suppressing the bath fluctuations and smutformation that arise due to elution of Al, Zn, and so forth, into theconversion treatment bath.

The invention is illustrated in greater detail hereinafter throughworking examples; however, the scope of the invention is not limited tothe following examples.

EXAMPLES Example 1

The surface of magnesium alloy sheet (type AZ91 ) was cleaned andsubjected to the following treatments.

1. Preparation of the Conversion Treatment Bath

An aqueous solution was prepared that contained 25 grams per liter(hereinafter usually abbreviated as "g/L") of 85% by weightorthophosphoric acid in water, 25 g/L of manganese dihydrogen phosphatetetrahydrate, and 20 g/L of triethylamine, with the balance being water.Its pH was 3.0.

2. Conversion Treatment

The specified magnesium alloy sheet was immersed in the aforementionedconversion treatment bath for 3 minutes at 40° C. to 45° C. It was thenwithdrawn, washed with water, and dried.

3. Tests on the Conversion Coating

(i) Inspection of the Conversion Coating

Using a microscope at 1000×, the surface of the conversion coating wasevaluated for the presence and magnitude of occurrence of reticulatinggrooves (cracks) and smut.

(a) Evaluation Scale for the Reticulating Grooves (Cracks)

++ presence of distinct reticulating grooves having widths of 0.1 to 2micrometers

+ presence of incomplete reticulating grooves having widths of 0.1 to0.5 micrometers

× absence of reticulating grooves, presence of a porous state

(b) Status of Smut Formation

++ no smut formation

+ formation of a relatively small amount of smut

× distinct smut formation

(ii) The phosphorus and manganese in the conversion coating werequantitatively analyzed by X-ray fluorescence (hereinafter usuallyabbreviated as "XRF").

(iii) Presence of Nitrogen Compounds in the Conversion Coating

The diffraction angles and intensities determined in X-ray diffractionwere compared with JCPDS cards (Hanawalt method).

4. Painting

A solvent-based acrylic paint (Saguran #3000 from Asahi Solvent Company)was sprayed onto the conversion coating--formed as described above--onmagnesium alloy sheet as described above (one coat, one bake, paint filmthickness=20 micrometers).

5. Salt-spray Testing (hereinafter usually abbreviated as "SST")

A cross was scribed into the painted panel obtained as described above,and salt-spray testing in accordance with JIS Z 2371 was then run on thepanel.

spray time: 120 hours

number of test panels: 50

After the exposure to salt spray, the larger of the blister width at thecross cut and the peel width at the cross cut after tape peeling wasselected and measured.

6.Water Resistance Tests

Checkerboard testing was conducted according to JIS K 5400.

temperature: 40° C.

time: 120 hours

number of test panels: 50

After exposure to the test conditions, the appearance and secondaryadherence were evaluated as described below.

(a) External Appearance

++ no blistering

+ minor blistering

× blistering

(b) Secondary Adherence

A 100-cell grid (10×10×1 mm) was executed according to JIS K 5400, andthe number of residual cells after tape peeling was measured.

Comparative Example 1

Treatment was conducted as in Example 1, except that the conversiontreatment bath contained 20 g/L of 85% orthophosphoric acid and 20 g/Lof triethylamine and did not contain manganese dihydrogen phosphate, andits pH was 5.0.

Comparative Example 2

Treatment was conducted as in Example 1, except that the conversiontreatment bath contained 25 g/L of 85% orthophosphoric acid and 25 g/Lof manganese dihydrogen phosphate tetrahydrate and did not containtriethylamine, and its pH was 2.0.

The test results from Example 1 and Comparative Examples 1 and 2 arereported in Table 1.

                  TABLE 1                                                         ______________________________________                                        CHARACTERISTICS OF THE CONVERSION COATINGS,                                   PAINT PERFORMANCE, AND OVERALL EVALUATION                                     FOR EXAMPLE 1 AND COMPARATIVE EXAMPLES                                        1 AND 2                                                                       Characteristic                                                                             Value or Rating of Characteristic for:                           Measured:    Example 1 Comp. Ex. 1                                                                              Comp Ex. 2                                  ______________________________________                                        Appearance:                                                                   Reticulation ++        ×    ++                                          Smut         ++        ++         ×                                     mg/m.sup.2 in                                                                 Coating of:                                                                   P            186       230        620                                         Mn            76       --         132                                         Nitrogen Compound                                                                          Yes       No         No                                          in Coating?                                                                   Water Resistance                                                              Test Results:                                                                 Appearance   ++        ×    ×                                     Sec. Adhesion                                                                              100       30-40      50-80                                       Salt Spray Test                                                                            1.0-1.5   2.0-4.0    2.0-3.0                                     Result, mm                                                                    Overall Evaluation                                                                         Excellent Poor       Poor                                        ______________________________________                                         Notes for Table 1                                                             "Comp. Ex." means "Comparative Example ". The nitrogen compound indicated     by Xray diffraction in the coating formed in the Example was manganese        nitride. "Sec." means "Secondary", and the values reported for secondary      adhesion are the number of squares, out of a total of 100 originally, to      which paint remained adhered after peeling; therefore, higher values are      preferred.                                                               

Comparative Example 3, Example 2, and Comparative Example 4

Comparative Example 3, Example 2, and Comparative Example 4 wereperformed according to the procedure of Example 1, except that theconversion treatment bath compositions were changed as shown in Table 2.Results from these examples are shown in Table 3.

                  TABLE 2                                                         ______________________________________                                        COMPOSITION OF CONVERSION TREATMENT BATHS                                                     Comp.               Comp.                                     Characteristic: Ex. 3    Example 2  Ex. 4                                     ______________________________________                                        g/L of 85% H.sub.3 PO.sub.4                                                                   25       35         25                                        g/L of Mn(H.sub.2 PO.sub.4).sub.2.4H.sub.2 O                                                  1.0      35         2.0                                       g/L of Triethylamine                                                                          1.0      28         24                                        pH              1.5      3.0        5.5                                       ______________________________________                                         Notes for Table 2                                                             "Comp. Ex." means Comparative Example. The balance of the treatment bath      not shown was water.                                                     

                  TABLE 3                                                         ______________________________________                                        CHARACTERISTICS OF THE CONVERSION COATINGS,                                   PAINT PERFORMANCE, AND OVERALL EVALUATION                                     FOR EXAMPLE 2 AND COMPARATIVE EXAMPLES                                        3 AND 4                                                                       Characteristic                                                                             Value or Rating of Characteristic for:                           Measured:    Example 2 Comp. Ex. 4                                                                              Comp Ex. 4                                  ______________________________________                                        Appearance                                                                    Reticulation ++        ++         ×                                     Smut         ++        ×    ++                                          mg/m.sup.2 in                                                                 Coating of:                                                                   P            286       477        85                                          Mn           115       4          7                                           Nitrogen Compound                                                                          Yes       No         Uncertain                                   in Coating?                                                                   Water Resistance                                                              Test Results                                                                  Appearance   ++        ×    ×                                     Sec. Adhesion                                                                              100       60         10                                          Salt Spray Test                                                                            1.0       2.5        3.5                                         Result, mm                                                                    Overall Evaluation                                                                         Excellent Poor       Poor                                        ______________________________________                                         Notes for Table 3                                                             The notes for Table 1 also apply to this table.                          

The invention claimed is:
 1. A process of forming a conversion coatingon a metal surface containing at least 55% by weight of magnesium bycontacting with said metal surface, for a time of 0.2 to 6 minutes at atemperature of 30° to 65° C., an aqueous liquid composition having a pHin a range from 2.0 to 5.0 and comprising water and:(A) a concentrationof a phosphorus-containing inorganic acid componant; (B) a concentrationof divalent manganese cations: and (C) a concentration of a component oforganic amines.
 2. A process according to claim 1, wherein component (C)is selected from the group consisting of aliphatic amines, heterocyclicamines and aromatic amines, all of said amines being soluble to theextent of at least 1% by weight at 25° C. in an aqueous solution havinga pH from 2.0 to 5.0.
 3. A process according to claim 2, whereincomponent (A) consists of orthophosphoric acid and component (B) isprovided by dissolving manganese dihydrogen phosphate, manganesehydrogen phosphate, or both in water to form the aqueous liquidcomposition.
 4. A process according to claim 3, wherein, in the aqueousliquid composition, the concentration of manganese in g-a/L has a ratioto the concentration of phosphorus in g-a/L that is from 0.10 to 0.30and the concentration of amine in M has a ratio to the concentration ofmanganese in g-a/L that is from 1.40 to 3.5.
 5. A process according toclaim 2, wherein, in the aqueous liquid composition, the concentrationof manganese in g-a/L has a ratio to the concentration of phosphorus ing-a/L that is from 0.10 to 0.30 and the concentration of amine in M hasa ratio to the concentration of manganese in g-a/L that is from 1.40 to3.5.
 6. A process according to claim 1, wherein, in the aqueous liquidcomposition, the concentration of manganese in g-a/L has a ratio to theconcentration of phosphorus in g-a/L that is from 0.10 to 0.30 and theconcentration of amine in M has a ratio to the concentration ofmanganese in g-a/L that is from 1.40 to 3.5.